This competing renewal examines the effect of chemoreceptor (CHEMO) and sympathetic nervous system (SNS) physiology in four groups of individuals: apneics and non-apneics, with and without hypertension. A total of 80 individuals will be studied in terms of their CHEMO and SNS function, sleep physiology, and quality of life. They will then be randomized to receive either nocturnal oxygen supplementation or nocturnal room-air supplementation and they will be restudied after 24 hours and after one week. Studies will include measurement of plasma and urinary catecholamines, beta adrenergic receptors on lymphocytes, plasma endothelin-1. CHEMO activity will be characterized while breathing room-air, while stimulating CHEMO with a brief period of hypoxia, and while blocking CHEMO with a brief period of hyperoxia. Using tritiated norepinephrine, norepinephrine release rate will be calculated during each of these CHEMO manipulations. Using impedance cardiography, cardiac hemodynamics will be characterized at rest and in response to behavioral stressors. The study will measure heart rate and tidal volume responses to infused isoproterenol. Baroreflexes will be characterized after stimuli that transiently increase and decrease blood pressure. Blood pressure will be characterized with ambulatory blood pressure monitoring. Sleep will be characterized with polysomnography. The effect of nocturnal oxygen treatment will be studied in terms of patient satisfaction, compliance, and quality of life.